Cellulose, which is the main component of paper substrates, is hydrophilic and polar. These characteristics result in rapid water penetration in the paper substrate. This phenomenon can be delayed by hydrophobation of the paper substrate. This operation is known as “paper sizing.” By this operation, the penetration of polar liquids (e.g., water or inks) in the paper is delayed by recovering the cellulose fibers with a hydrophobic substance, which is called a sizing agent. The sizing agent is usually added to the paper pulp and retained by the cellulose fibers in the wet end part of the paper manufacturing process. This is called internal sizing. However, the sizing agent can also be applied to the surface of the dried or partially dried paper, i.e., surface sizing.
Typical sizing agents include rosin, alkenyl succinic anhydrides (“ASAs”) and alkyl ketene dimers (“AKDs”). ASAs are good candidates for surface sizing of paper substrates due to their high reactivity towards the hydroxyl groups of the cellulose. The reaction between ASAs and cellulose can be represented as follows:
The formation of covalent bonds between the cellulose and the ASA translates into an efficient sizing and the resulting paper products show a good resistance to polar liquids penetration.
For surface sizing applications, the ASA is generally emulsified in water and the emulsion is applied using a sizing press or a coater. ASA emulsion can also, but less often, be applied using a shower. However, some curl problems have been observed when applying ASA water emulsions using showers. Moreover, for efficiency reasons and to obtain a sizing as homogeneous as possible, the particle size has to be controlled and a limited particle size range must be obtained. The water emulsion containing ASA must also be used quickly so as to limit ASA's hydrolysis, which produces a product interfering with sizing.
Methods have been proposed to prevent ASA's hydrolysis. For example, a water emulsion containing cationic starch is prepared as late as possible before application. However, the process for mixing the ASA with cationic starch is rather complicated and uses a complex machine.
Another known hydrophobation method used to increase paper water resistance is vapor depositing the ASA on the paper surface. In this case, ASA in gaseous phase contacts the paper surface and reacts with the hydroxyl groups of the cellulose. However, applying such a method at an industrial scale would imply using a complex system in order to avoid releasing ASA in the atmosphere surrounding the machine. A complicated system would be required for confining the molecules in gaseous phase while allowing their contact with the paper sheet in continuous movement.
In light of the aforementioned, there is thus a need for new ASA containing sizing compositions as an alternative to ASA water based compositions.